Polyurethanes have been used for a wide variety of applications. Since the early 1970's aqueous polyurethane dispersions have expanded in utility. The incorporation of ionic salt groups into the polyurethane prepolymer before dispersing into water greatly facilitates the formation of the aqueous polyurethane dispersion. Aqueous polyurethane dispersion technology has matured considerably; however, the basic polyurethane in the polyurethane dispersion is the result of reacting a diisocyanate, a diol or diamine (polyether, polyester, etc.) and a hydroxyl or diol (or amine analogue) molecule containing an ionic salt group. The ionic salt groups most often used are carboxylic, sulfonic and phosphoric acids (or the base neutralized acids), amine (or acid neutralized amine), or quaternary nitrogen groups. One of the most often used carboxyl reactants is dimethylol propionic acid.
Aqueous polyurethane dispersions have been used extensively as adhesives and coatings on essentially any synthetic or natural substrate such as paper, wood, cotton, leather, wool and hair. Specific applications in the pharmaceutical, cosmetic and hair fixative areas have been disclosed. Recently hair fixative applications have focused upon using hard polyurethanes which have a glass transition, Tg, above about room temperature. These hard, high Tg polyurethanes are used to provide the hold and curl retention necessary for hair fixatives but suffer from unpleasant, hard aesthetic feel on hair and brittleness of the polyurethane which results in flake development, raspy feel, or difficult to comb properties.
Most hair fixative compositions contain a film-forming polymer, which acts as the fixative, and a delivery system, which is usually an alcohol or a mixture of alcohol and water. In the case of aerosol delivery, the delivery system will also contain a propellant, which is typically a volatile hydrocarbon. Due to environmental regulations controlling the emission of volatile organic components (VOCs) into the atmosphere, these alcohol and hydrocarbon delivery systems are becoming less acceptable, and it is foreseen that water will become a greater component in hair fixative compositions. In addition, several different delivery systems for hair fixatives are now utilized, for example hair sprays, both aerosols and pumps, gels and mousses. Hair fixative polymers taught for use in aqueous based systems are known, for example, those disclosed in Japanese publication JP 47-46332. However, many of these exhibit a loss of performance properties in aqueous systems, for example, curl retention and on-hair stiffness are inferior, and in other cases the solution viscosity increases, and if delivered by aerosol, the composition foams at the valve actuator and on the hair. In addition, current systems may also exhibit poor compatibility with aqueous delivery systems. Overall the requirements for hair fixatives have increased. Performance requirements for hair fixatives now demand that the hair fixative polymer maintain a high degree of hold, gloss, curl retention, stiffness, and humidity resistance, but yet have pleasing aesthetics with a natural soft feel with no adhesive tackiness, no raspiness or brittle feel, no flake development and yet be readily removable. These factors have prompted the search for better performing hair fixative polymers that are soluble or dispersible in aqueous or in low VOC systems, that is, systems containing 80% or less VOCs. Current systems do not provide this required balance of properties.
U.S. Pat. No. 5,626,840 discloses polyurethanes prepared from an organic diisocyanate, a diol with a number average molecular weight greater than 1000, and a 2,2-hydroxymethyl-substituted carboxylic acid which are neutralized with a cosmetically acceptable organic or inorganic base and formulated into a hair fixative composition containing low amounts of volatile organic solvent.